Modern printing devices, for example, ink jet and laser printers, print on a wide range of print media. Such media include plain paper, glossy or coated papers, and plastic films including overhead transparency film. For optimal print quality, operating parameters of these printers may be adjusted to meet the requirements of each print medium. Parameters in the image rendering process, in a host computer or in an "on-board" computing engine in the printer, also depend upon media type. For example, the "gamma" (i.e., tone reproduction curve) used for reflective prints (on paper and other reflective media) is different than that used for transparency media. This is required to adapt the printed image to the characteristics of the human visual response under different lighting and viewing conditions. Therefore, both the recording process in the printer and the image rendering process, in a host computer or on-board computing engine, may require knowledge of media type for optimal print quality.
The software controlling the rendering process and the printer, including the printer driver, sometimes gives the user the opportunity to specify the recording medium. Parameters of the rendering and recording processes are then adjusted according to the recording medium and the quality mode selection. However, users may not always make the correct choice. In addition, specifying the choice is often inconvenient when multiple copies on different media are desired as occurs when overhead transparencies and hardcopy for handouts must be produced from the same data file.
One approach to this problem is to use recording media marked by machine-readable, visible, near-visible, or invisible marks forming bar codes or other indicia that specify media type and automatically provide process information to the printer. While this offers a practical solution, not all media available to the user will contain these codes.
Other approaches known in the art distinguish between two broad classes of media, transparency film and paper. For example, U.S. Pat. No. 5,139,339, to Courtney et al. discloses a sensor which measures diffuse and specular reflectivity of print media to discriminate between paper and transparency film and to determine the presence of the print medium. Other art cited in Courtney et al. deals mainly with analyzing specular reflections over an area. U.S. Pat. No. 5,323,176 to Suguira et al. describes a printer with means to discriminate between "ordinary printing paper" and "overhead projection transparency film" on the basis of its transparency or opacity. However, these references, which rely on gross properties of the print medium either in reflection or transmission, do not allow any finer distinctions. What is needed is a method to distinguish print media that goes beyond the simple categorizations of the prior art.